Frequency tuning for modulated plasma systems

1. A power generation system comprising:

a high-frequency generator configured to apply power to a plasma chamber at a primary frequency;

a filter configured to couple to an output of the high-frequency generator to suppress mixing products to limit variation of a time-varying load reflection coefficient presented to the high-frequency generator; and

a frequency-tuning subsystem configured to:

apply, while the high-frequency generator is applying power at the primary frequency, a probe signal comprising one or more probe frequencies, wherein the one or more probe frequencies are different than the primary frequency, wherein power of the probe signal is lower than power produced at the primary frequency; and

adjust the primary frequency of the high-frequency generator in response to the one or more probe frequencies indicating an improved measure of performance.

2. The power generation system of claim 1 , comprising a low-frequency generator to apply power to the plasma chamber at a low frequency, wherein the filter is configured to suppress mixing products of the primary frequency and the low frequency.

3. The power generation system of claim 2 , wherein a frequency ratio of the low-frequency generator to that of the high-frequency generator is between 0.0005 and 0.2.

4. The power generation system of claim 1 , wherein power produced at the probe frequencies is between 1 and 100 dB below the power at the primary frequency.

5. The power generation system of claim 1 comprising a delay element configured to couple between the filter and the plasma chamber.

6. The power generation system of claim 1 , wherein the measure of performance is a measure of performance selected from the group consisting of: a reflected power; a measure of how far a load impedance seen by the high-frequency generator deviates from a desired impedance; and a measure of load reflection coefficient magnitude.

7. The power generation system of claim 1 , wherein the one or more probe frequencies are noise.

8. The power generation system of claim 7 comprising a single oscillator-amplifier combination to produce the primary power signal, and the noise is inherent to the single oscillator-amplifier combination.

9. The power generation system of claim 1 , comprising a primary oscillator to generate the primary power signal and a secondary oscillator to generate the one or more probe frequencies.

10. A method for automated frequency tuning of a power generation system, the method comprising:

applying a primary power signal at a primary frequency to a plasma load with a high-frequency generator, either directly or through a matching network;

applying a probe signal at one or more probe frequencies to the plasma load, wherein the one or more probe frequencies are different than the primary frequency, and wherein power produced at the probe frequencies is lower than power produced at the primary frequency;

suppressing mixing products with a filter that is separate from the high-frequency generator to reduce variation of a time-varying load reflection coefficient presented to the high-frequency generator; and

adjusting the primary frequency based upon a measure of performance in response to probe signal.

11. The method of claim 10 , wherein the application of the probe signal and the adjustment of the primary frequency are performed cyclically in order to repeatedly improve an accuracy of adjusting the primary frequency towards a global optimum.

12. The method of claim 10 , wherein the probe signal is periodic or a sum of periodic signals.

13. The method of claim 10 , wherein applying the probe signal comprises sweeping the one or more probe frequencies across a fixed frequency range.

14. The method of claim 10 , wherein applying the probe signal comprises tuning a probe signal to a single one of a plurality of different probe frequencies at different times.

15. A plasma processing system comprising:

a plasma chamber;

a high-frequency generator configured to apply power to a plasma chamber at a primary frequency;

a low-frequency generator to apply power to the plasma chamber at a low frequency;

a filter coupled between the high-frequency generator and the plasma chamber that is configured to suppress mixing products of the primary frequency and the low frequency to limit variation of a time-varying load reflection coefficient presented to the high-frequency generator; and

means for frequency tuning the high-frequency generator using a probe signal that is concurrently applied with the power applied to the plasma chamber at the primary frequency wherein the probe signal comprises one or more probe frequencies that are different than the primary frequency.

16. The plasma processing system of claim 15 comprising a match network coupled between the high-frequency generator and the plasma chamber.

17. The plasma processing system of claim 15 , wherein a frequency ratio of the low-frequency generator to that of the high-frequency generator is between 0.0005 and 0.2.

18. The plasma processing system of claim 15 , wherein power produced at one or more probe frequencies of the probe signal is between 1 and 100 dB below the power at the primary frequency.

19. The plasma processing system of claim 15 wherein the means for frequency tuning comprises means for frequency tuning with a probe signal that comprises noise.

20. The plasma processing system of claim 15 , wherein the means for frequency tuning comprises a secondary oscillator, and wherein the high-frequency generator comprises a primary oscillator to generate the primary frequency and the secondary oscillator to generate the probe signal.